I have told several members off list about Amaco
Engobes (slips). For those of you without all of the
ingredients for making your own slips - try these
Engobes/slips from Amaco. They are still quite
affordable while underglazes (that can also work for
sgraffito) have gone up considerably. Dark blue is
really more of a medium blue however (I have a pot I
can share that shows it fired with a clear glaze
over).

Natural black is a brownish black -- jet black is a
bluer black (I have a pot with I will photograph).
White is also very opaque and works well with
sgraffito and mishima techniques. I also used the red
brown. I did not use the dark green at all.

These are available from Sax and many other art supply
catalogs...as well as directly from Amaco. They take
the guess work out. I would still recommend doing some
test tile samples before using them with kids. Buy one
of each color you are considering and experiment. They
come in dry form...you can only mix a little at a
time...They are lead free and give opaque coverage.
Dark Blue does have a "cautionary label" - probably
becaseu of cobalt. Where a mask when mixing.

My Sax catalog is old -- they are on page 364 in mine
- so look around where the glazes are listed.

Another easy way to get some basic slips is to have a
professor at a nearby college make some for
you(smile). I had a professor at Ohio Northern make
some for me - cost very little money (he may have done
them for free?). I used them for years at the local
art resource center. I had him make white, black and
blue. Pick up the phone - it is worth a try.